Matsubara Toshiaki, Hirao Kazuyuki
Institute for Fundamental Chemistry, 34-4 Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan.
J Am Chem Soc. 2002 Jan 30;124(4):679-89. doi: 10.1021/ja0113495.
The mechanism for the activation of the sigma bonds, the O-H of H2O, C-H of CH4, and the H-H of H2, and the pi bonds, the C[triple bond]C of C2H2, C=C of C2H4, and the C=O of HCHO, at the Pd=X (X = Sn, Si, C) bonds of the model complexes (H2PC2H4PH2)Pd=XH2 5 has been theoretically investigated using a density functional method (B3LYP). The reaction is significantly affected by the electronic nature of the Pd=X bond, and the mechanism is changed depending on the atom X. The activation of the O-H bond with the lone pair electron is heterolytic at the Pd=X (X = Sn, Si) bonds, while it is homolytic at the Pd=C bond. The C-H and H-H bonds without the lone pair electron are also heterolytically activated at the Pd=X bonds independent of the atom X, where the hydrogen is extracted as a proton by the Pd atom in the case of X = Sn, Si and by the C atom in the case of X=C because the nucleophile is switched between the Pd and X atoms depending on the atom X. In contrast, the pi bond activation of C[triple bond]C and C=C at the Pd=Sn bond proceeds homolytically, and is accompanied by the rotation of the (H2PC2H4PH2)Pd group around the Pd-Sn axis to successfully complete the reaction by both the electron donation from the pi orbital to Sn p orbital and the back-donation from the Pd dpi orbital to the pi orbital. On the other hand, the activation of the C=O pi bond with the lone pair electron at the Pd=Sn bond has two reaction pathways: one is homolytic with the rotation of the (H2PC2H4PH2)Pd group and the other is heterolytic without the rotation. The role of the ligands controlling the activation mechanism, which is heterolytic or homolytic, is discussed.
利用密度泛函方法（B3LYP）对模型配合物（H2PC2H4PH2）Pd=XH2 5（X = Sn、Si、C）中Pd=X键（X = Sn、Si、C）处σ键（H2O的O-H、CH4的C-H以及H2的H-H）和π键（C2H2的C≡C、C2H4的C=C以及HCHO的C=O）的活化机制进行了理论研究。该反应受到Pd=X键电子性质的显著影响，且反应机制随原子X的不同而改变。具有孤对电子的O-H键的活化在Pd=X（X = Sn、Si）键处是异裂的，而在Pd=C键处是均裂的。没有孤对电子的C-H和H-H键在Pd=X键处也被异裂活化，与原子X无关，其中当X = Sn、Si时氢被Pd原子以质子形式提取，当X = C时氢被C原子提取，因为亲核试剂根据原子X在Pd和X原子之间切换。相反，Pd=Sn键处C≡C和C=C的π键活化是均裂进行的，并且伴随着（H2PC2H4PH2）Pd基团围绕Pd-Sn轴的旋转，通过π轨道向Sn p轨道的电子给予以及Pd dπ轨道向π轨道的反馈给予成功完成反应。另一方面，Pd=Sn键处具有孤对电子的C=O π键的活化有两条反应途径：一条是伴随着（H2PC2H4PH2）Pd基团旋转的均裂途径，另一条是没有旋转的异裂途径。讨论了控制活化机制（异裂或均裂）的配体的作用。
